During operation, automotive turbocharger housings are subjected to elevated operating temperatures. These housings must be able to contain a turbine wheel rotating at very high speeds. Exhaust gas from the automotive initially contacts the turbocharger in metal sections, such as the gas inlet area of the turbocharger, at elevated temperatures. As high-speed performance improves through exhaust temperature increase, there have been attempts to gradually raise the exhaust temperature of the engine. Due to these high temperatures, the thermal load on the parts such as the exhaust manifold and the turbine housing becomes very great.
Various problems have been encountered by these increased exhaust gas temperatures contacting metal sections of the turbocharger. For example, one problem caused by the exhaust temperature rise is the problem of thermal deformation of the material, wherein the turbine housing and exhaust manifold, which alternates between regions of high temperature and low temperature is accompanied by thermal expansion and contraction depending on the situation, which can cause surface oxidation, and develop into a crack or other failure mode.
In order to overcome the challenges associated with higher operating temperatures, prior art alloys used in turbocharger applications have included alloys of higher nickel content such as commercially available high nickel ductile iron casting alloys. Examples of these are NiResist™ developed by the International Nickel Company, or HK40, a chromium-nickel-iron stainless steel alloy containing approximately 25% chromium and 20% nickel, with the balance essentially iron. The HK series stainless steel alloys, such as the HK40 alloy noted herein, in general have about 18-22% nickel and are fully austenitic.
The HK stainless steel alloys are strong stainless steel casting alloys, in terms of creep strength. However, while meeting the high temperature property requirements for turbocharger housings, they are quite expensive because of their high nickel content. Further, due to the sudden rise of the Ni price recently, there has been substantial concern over the cost of materials that have a relatively high nickel content.
For lower temperature applications, ferritic stainless steel GX40 is currently recommended for operation until 900° C., beyond which austenitic materials such as NiResist and HK40 are preferred. Due to the high nickel content of these austenitic steels, a ferritic steel alternative is cheaper and attractive. The application of commercially available ferritic stainless steels such as GX40 is limited to temperatures below 900° C. due to less stable ferrite phase and lower oxidation resistance beyond this temperature. Any modification to the chemistry, which can improve the ferrite phase stability and oxidation resistance and thus increase the operating temperature will be present a cheaper alternative to austenitic grades.
Accordingly, there is a need for stainless steel alloys useful in turbocharger applications that are able to withstand the higher operating temperatures produced by modern engines, but that minimize the expensive nickel content. Ideally, a ferritic stainless steel would be employed that does not include any nickel content at all. Furthermore, other desirable features and characteristics of the inventive subject matter will become apparent from the subsequent detailed description of the inventive subject matter and the appended claims, taken in conjunction with the accompanying drawings and this background of the inventive subject matter.